U.S. Pat. No. 5,601,307 discloses a vehicle suspension system in which a double-acting hydraulic ram supports the vehicle at each wheel with the ram chambers at opposite diagonal comers of the vehicle cross-connected so as to provide four discrete hydraulic wheel circuits. A load distribution unit is connected to the four hydraulic circuits for hydraulically equalizing the pressures in the circuits. FIG. 5 of U.S. Pat. No. 5,601,307 discloses a load distribution unit (LDU) 140 which includes a pair of cylinders, each containing a double-acting ram that divides the cylinder which contains it into four variable volume chambers, one chamber at each end of the ram and two chambers inward of the two end chambers, each chamber being isolated from all the others. Each of the inward chambers communicates with a different one of the wheel circuits, with each LDU cylinder being connected to the same chambers of the wheel cylinders on opposite sides of the vehicle, i.e. one of the inward chambers of one of the LDU cylinders is connected to the wheel cylinder circuit that connects the front left bore side (upper) wheel cylinder chamber with the rear right rod side (lower) wheel cylinder chamber, and the other inward chamber of the same LDU cylinder is connected to the wheel circuit that connects the front right bore side wheel cylinder chamber with the rear left rod side wheel cylinder chamber. In the same manner, the inward chambers of the other LDU cylinder are connected to the two wheel circuits that connect the front rod side wheel cylinder chambers with the rear bore side wheel cylinder chambers. A total of six solenoid-operated pressure supply valves 204 are supplied for providing hydraulic fluid under pressure to the various chambers of the LDU, and a total of six pressure relief or drain valves 215 are provided for relieving pressure from the various chambers. Six valves are needed in each valve bank because the two end chambers (sometimes referred to as "bump" chambers) of each LDU cylinder are connected to the corresponding two end (bump) chambers of the other LDU cylinder. As a result, a pressure-tank valve pair is needed for each of the four wheel cylinder circuits and a pair is needed for each of the two connected end (bump) chambers, for a total of six pressure-tank valve pairs (12 valves total). In addition, a reservoir 66 is provided for the hydraulic fluid, together with a pump 65 and gas-charged accumulators 202 for maintaining pressure in the system.
The LDU equalizes pressures between the various wheel circuits, and also provides a means of adding fluid to the various discrete circuits or taking it away, for load leveling and raising or lowering of the vehicle relative to the wheels. The LDU contains two pistons, which have sliding seals and must be very low leak and low friction. Excess leakage in the seals or valves results in having to run the pump excessively, or the vehicle losing pressure too quickly when parked, with consequent unintended changes in attitude or level of the vehicle relative to the wheels. Competing with the need to make the seals tight and therefore with good leakage characteristics is the need to provide low friction in the movement of the LDU pistons. High friction results in poor performance of the LDU, and unintended and unpredictable differences in the pressures between the circuits of the system, and sudden changes in pressures occurring when the LDU pistons break lose of static friction binding them and start moving.
In addition, the linear position of the LDU pistons is monitored by the onboard computer. A linear position transducer must be provided for this purpose for each LDU piston, which provides a challenge to fit it into the envelope provided and to contain the pressures required.